Acid base / electrolyte - General

Question 1

Formula for expected CO2 in metabolic acidosis

Winters formula

Answer 1

PaCO2 = 1.5 x HCO3 + 8

+/- 2

Question 2

Formula for expected CO2 in metabolic alkalosis

Answer 2

PaCO2 = 0.7 x HCO3 + 20

+/- 5

Question 3

Formula for expected bicarb in acute respiratory acidosis

Answer 3

HCO3 = 24 + (PaCO2 - 40) / 10

Question 4

Expected bicarb in chronic respiratory acidosis

Answer 4

HCO3 = 24 + (PaCO2 - 40)/10 x 4

Question 5

Calculation for anion gap

Answer 5

Anion gap = (Na + K) - (Cl + HCO2)

16 +/- 4

Question 6

Albumin adjustment for anion gap

Answer 6

= anion gap + 0.25 x (40-albumin g/L)

Question 7

Causes of HAGMA

Answer 7

CATMUDPILES

• CO, Cyanide
• Alcoholic ketoacidosis / starvation
• Toulene
• Metformin, methanol
• Uraemia
• DKA
• Pyroglutamic acidosis: paracetamol, paraldehyde
• Iron, isoniazid
• Lactic acidosis
• Ethylene glycol
• Salicylates

Question 8

Causes low anion gap

Answer 8

• Hypoalbuminaemia
• Calculation error
• Hyperchloraemia
• Increased cations: calcium, magnesium, LITHIUM, SODIUM, hyperproteinaemia (Ig)
• Bromide / iodine poisoning (falsely elevated Cl-)

Question 9

Causes of normal anion gap metabolic acidosis

Answer 9

RACE

• Renal: RTA, addisons
• A: Acetazolamide
• C: chloride excess (saline)
• Extra loss: diarrhoea, fistula

Question 10

Formula for delta ratio

Answer 10

Change AG / Change bicarb

= AG - 12 / 24-HCO3

Question 11

How do you interpret the anion gap

Answer 11

AG < 0.4 - NAGMA
AG 0.4-0.8 - mixed
AG 1-2 HAGMA
>2 HAGMA and met alkalosis OR pre-existing chronic resp acidosis

Question 12

Causes of metabolic alkalosis

Answer 12

CLEVER PD
Contraction (dehydration) 
Liquorice / Laxatives 
Endocrine - conn’s Cushing 
Vomiting / GI loss 
Excess alkali, Antacids, calcium, bicarb, ural, citrate 
Renal (Bartter, gitelman’s syndrome)
Post hypercapnoea 
Diuretics

Question 13

Causes of respiratory alkalosis

Answer 13

Increased Aa gradient
- hypoxia, lung disease 
- Cardiac failure 
- Sepsis 
Normal Aa gradient 
- hypoxia, altitude, low FiO2 
- Progesterone (pregnancy)
- Theophylline, salicylate, stimulants 
- Cerebral oedema
- Hepatic encephalopathy
- Mechanical ventilation
- Anxiety

Question 14

Describe the grading of hyponatraemia

Answer 14

Mild 130-135
Moderate 125-129
Severe <125
< 115 - Significant neurological problems

Question 15

Causes of SIADH

Answer 15

Tumours - lung, leuk/lymphoma
Neurological - trauma, infection, SAH, AIDS
Pulmonary - pneumonia, abscess, PPV, TB
Drugs 
- Serotinergic 
- Haloperidol 
- Omeprazole
- Carbamazepine
- Cyclophosphamide, vincristine 
Idiopathic

Question 16

How to calculate sodium deficit

Answer 16

Na = (125-Na) x TBW
TBW = 0.6 x BW (reduce in elderly / females)

Question 17

Causes of hypernatraemia

Answer 17

Iatrogenic - Na, bicarb
Extra-renal hypotonic fluid loss: burns, exposure, GIT loss
Renal loss - osmotic diuretic
Diabetes insipidus (iatrogenic, neurogenic, gestational, nephrogenic)

Question 18

Describe classification of hyponatraemia

Answer 18

Hyper / hypo / iso osmotic 
Hypo-osmotic 
Urine osmolality 
High. 
Urine Na+
- High (Na wasting, H2O retention) 
- Low (Na & H2O retention) 
Low osmolality
Urine Na+ 
- High (Na & H2O wasting)
- Low (Na retention and normal H2O response)

Question 19

Calculation of free water deficit

Answer 19

Na - 140 / 140 x TBW

TBW ~ 0.6 x BW